102 Of the Semen Mafculinum, 
microfcope, great numbers of animals were feen fwim- 
ming therein in legions, and crofting one another like 
clouds in a ftormy day, as brifk as if the cock was qewly 
dead u , although it was killed the day before; they appear 
as at fig. 168. if viewed with due attention, and with the 
greateft magnifiers, otherwife only in the form of eels. 
Mr. Leeuwenhoek, in the fpring- time, when the frogs 
engender, opened the tefticles of the male x , and on ap^ 
plying fome of the feminal matter to the microfcope, 
multitudes of animalcules appeared therein, about one 
thoufandth part of the thicknefs of a human hair; and 
there feemed to be ten thoufand of them at leaft to each 
one of the female ova; their form is as reprefented ia 
fig. 169. 
Mr. Leeuwenhoek’s method of computing the fize of 
animalcules was this, he placed an hair y of his head 
near them, which hair appeared an inch in breadth ; and 
being fatisfied that fixty of the animalcules could lie 
within that diameter; whence their bodies being fphe- 
lical, 216,000 of them are but equal to a globe, whofe 
diameter is no more than the breadth of fuch an hair. 
Another method of his alfo follows. 
He firft fuppofed a drop of water equal to a pea; then 
took a little quantity of water, of a round figure, as big 
as a millet grain; and reckoned this to be one ninety-one 
part of a pea z ; for when the axis of a millet feed makes 
one, that of a pea will make four and half, whence it 
follows, that the feed of a millet is at leaft the one ninety- 
one part of a pea; this fmall quantity of water he put 
into a very flender glafs tube, dividing by this means that 
little water into twenty-five or thirty parts, and found 
more 
111 Arc. Nat. Tom. ii. Part. ii. p. 369. x Arc. Nat. 
Tom. i Part. i. p. 51. y Phil. Tranf. No. 270. z Ibid, 
No. 131. 
